Metabolism provides the vital energy and building blocks necessary for the physiological function of all cells. The brain is one of the most metabolically demanding tissues, and cellular metabolic dysfunction is a hallmark of devastating neurological conditions ranging from epilepsy to Parkinson's Disease. Although the components of the core metabolic pathways have been known for decades, little is known about their moment-to-moment behavior and interactions in living cells. To better understand why metabolic dysfunction occurs in diseased states, it is crucial to first establish how energy metabolism of brain cells behaves in vivo. Traditional approaches to study cellular metabolism lack the temporal and spatial resolution required to examine real-time metabolism in native tissues. These barriers can be overcome by using genetically-encoded fluorescent biosensors. Recently created sensors that report the ATP/ADP ratio and the NADH/NAD+ ratio enable us to monitor energy metabolism in living brain cells. The cytosolic ATP/ADP ratio reflects the overall energetic state of the cell, while the cytosolic NADH/NAD+ ratio provides a read-out of glycolytic activity. Monitoring these two key metabolites in vivo will provide us unprecedented insight into the metabolic activities of brain cells during resting and active conditions in the intact brain. Te proposed research combines the use of fluorescent ATP and NADH biosensors with advanced optical imaging techniques to study the energy metabolism of neurons and astrocytes in vivo. The goal of Aim 1 is to characterize the dynamics of NADH and ATP levels in these cell types in vivo during resting and active states. This work will also examine whether glycolysis is activated in both cell types in response to physiological neuronal activity. The goal of Aim 2 is to determine whether NADH and ATP levels in neurons and astrocytes in vivo are altered in a mouse model where glucose is not the preferred mitochondrial fuel. This mouse model displays seizure resistance and characterizing its cellular neural metabolism may provide valuable insight into how metabolic modulation can regulate neuronal excitability.